This invention relates to a method and apparatus used to compensate for thermal displacement. More particularly, the invention relates to a method and apparatus which enable, a spindle positioning error attributed to thermally induced displacement to be compensated for in a simple manner.
In a machining center of the like a spindle holding a tool is, rotated at high speed and is at the same time, moved toward a workpiece along the Z-axis to position the tool with respect to the workpiece. The workpiece, on the other hand, is transported relative to the spindle along the X- and Y-axes so that it may be machined by the tool into the shape specified by the machining program. It is obvious then, that any positional error along the Z-axis will make it impossible to achieve a high degree of machining accuracy.
In machine tools of the foregoing type a DC motor is employed to rotate the spindle. In DC motor structures an armature winding is wound on approximately the central portion of a spindle, and a field winding for generating a magnetic flux is disposed around the armature winding. When the spindle is rotated the armature and field windings produce heat, as do the bearings supporting the spindle. The total amount of such heat is enough to cause as much as 40 to 50 microns of spindle elongation, the result being a decline in positional accuracy along the Z-axis. Until now highly precise machining has not been possible because of such displacement along the Z-axis.